Glass microspheres embedded in the surface of painted lines and other pavement markings to serve as retroreflective elements are subjected to extremely destructive conditions, the most common being the myriad number of impacts which the microspheres receive from the wheels of passing vehicles. Such impacts tend to eventually crush and abrade the microspheres, thereby reducing or taking away the transmission of light that is necessary for retroreflection.
This problem has been compounded by the fact that glass microspheres having the higher indexes of refraction best adapted for retroreflectivity in pavement markings (i.e., above 1.5) have been especially susceptible to crushing and abrading. Such higher-index microspheres have had only limited use in pavement markings, which has significantly reduced the reflectivity available from pavement markings.
Glass microspheres in pavement markings are also subject to attack by corrosive agents sometimes present in the air, such as acids formed by various emissions from automobiles, furnaces, industrial plants, etc. Such attack can cause the microspheres to lose transparency, which also reduces or takes away their reflectivity.